1. Field of the Invention
The present invention relates to a device for confirming the opening of a welding gun, i.e., a spaced-apart condition of welding electrodes thereof, used in spot welding, resistance welding, or the like, and more particularly to a device for confirming the opening of a welding gun, the device having a reciprocating mechanism for moving a movable electrode into and out of contact with a workpiece.
2. Description of the Related Art
Resistance welding is a welding process in which a large electric current is passed through joint areas of workpieces to melt the joint areas with the heat produced from resistance of the workpieces, and a pressure is applied to join the melted joint areas to each other. The resistance welding processes are largely classified into lap resistance welding and butt resistance welding.
The lap resistance welding is a welding process which welds thin overlapping metal plates to each other. The lap resistance welding processes include a spot welding process, a projection welding process, and a seam welding process. According to the spot welding process, the two overlapping workpieces are sandwiched between electrodes, and an electric current is passed through joint areas of the workpieces while they are kept under pressure for locally heating the joint areas, producing coalescence of the joint areas with the generated heat thereby to weld the joint areas to each other. According to the projection welding process, protrusions of the joint areas of the workpieces are held against each other, and an electric current is passed through the joint areas while they are kept under pressure for thereby welding the joint areas at several spots in one welding cycle. According to the seam welding process, thin metal plates are continuously welded by roller electrodes which pass an electric current through the thin metal plates while holding them under pressure.
The butt resistance welding is a welding process for welding together two wires or rods which are held end to end against each other in alignment. The butt resistance welding processes are divided into an upset welding process, a flash welding process, and a butt seam welding process. According to the upset welding process, the ends of two workpieces are held against each other, and joined together by the Joule heat obtained when an electric current flows through the workpieces and under a pressure applied to the workpieces. According to the flash welding process, the ends of two workpieces are not strongly pressed against each other in an initial stage of the application of an electric current, causing the contacting ends to be melted and scattered as sparks while heating the joint areas to a sufficiently high temperature, and then the joint areas are strongly pressed against each other, so that they are welded to each other fully across their surfaces. According to the butt seam welding process, pipe ends are welded to each other to produce a pipe seam.
Welding machines, generally called welding guns, for carrying out the resistance welding have a reciprocating mechanism for moving a movable electrode into and out of contact with workpieces, and a welding transformer for passing an electric current through the movable electrode and a fixed electrode which are held against the workpieces.
When a resistance welding process, e.g., a spot welding process, is carried out on workpieces by a welding gun, the workpieces are welded at several points thereon by the welding gun. At this time, movable and fixed electrodes are mounted on respective arms that are movable vertically and horizontally, the movable and fixed electrodes having confronting electrode faces. The workpieces are welded by the movable and fixed electrodes at successive points thereon while the arms are being intermittently moved. Alternatively, the arms are fixed, and a table with the workpieces fixedly mounted thereon is moved while the workpieces are welded by the movable and fixed electrodes at successive points thereon.
In the spot welding process, after it has been confirmed that the welding at a point is fully completed, a next point is welded. The completion of the welding at each point is confirmed by confirming the welding-gun-opening displacement of a piston of a piston-actuated reciprocating mechanism which moves the movable electrode, i.e., the state in which the movable and fixed electrodes are spaced a maximum distance from each other. In this manner, it is confirmed that the welding gun is completely opened before the next point is welded, for thereby avoiding an operation failure (caused by incomplete opening of the welding gun) due to sticking together of electrode tips at the time of welding the next point.
One conventional device for confirming the welding-gun-opening displacement of the piston comprises a displacement detector positioned between a pair of gun arms for detecting a displacement of the gun arms to detect the open state of the welding gun, as disclosed in Japanese laid-open utility model publication No. 48-25014. Another device for confirming the welding-gun-opening displacement of the piston, as disclosed in Japanese laid-open utility model publication No. 1-151986, comprises a displacement detector such as an optical distance sensor mounted in a pressurizing cylinder for detecting the position of a piston of the pressurizing cylinder thereby to confirm the welding-gun-opening displacement of the piston.
The device revealed in the latter publication is advantageous in that since the optical distance sensor is mounted in the pressurizing cylinder, it is not subject to a magnetic field generated by an electric current flowing through the gun arms, and hence is capable of accurately detecting the opening of the welding gun even when the electric current flows through the gun arms. The optical distance sensor which detects the welding-gun-opening displacement of the piston in a contactless manner is free of wear-induced damage or failure that would otherwise happen to contact-type detectors. However, the optical distance sensor directly mounted in the pressurizing cylinder suffers various shortcomings described below.
In the pressurizing cylinder which moves the movable electrode, the piston is axially displaced under a fluid pressure such as a pneumatic pressure or the like. Therefore, dust or dirt particles or a mist in the cylinder tend to float and be deposited on the photodetector surface of the optical distance sensor. When this occurs, the photodetector surface becomes blurred by the deposited matter, lowering the sensitivity (detecting capability) of the optical distance sensor.
When the piston is first inserted into the cylinder in an assembling procedure, a sliding lubricant such as grease or the like is coated on the inner surface of the cylinder. The coated grease is apt to be applied to the photodetector surface, also lowering the sensitivity of the optical distance sensor.
To avoid the above drawbacks, it is necessary to check the sensitivity of the optical distance sensor after it has been installed in the cylinder. If the sensitivity of the optical distance sensor is found not sufficiently high, then the optical distance sensor has to be removed and installed again for desired sensitivity.
The process of checking the sensitivity of the optical distance sensor and possibly reinstalling the optical distance sensor makes the assembling procedure complex and time-consuming.